1. Field of the Invention
This invention relates generally to the field of therapeutic drug delivery, and in particular to the delivery of therapeutic liquids to the respiratory system.
A wide variety of procedures have been proposed to deliver a drug to a patient. Of particular interest to the present invention are drug delivery procedures where the drug is in liquid form and is delivered to the patient's lungs. Effective intrapulmonary drug delivery depends on a variety of factors, some of which can be controlled by the clinician or scientist and others that are uncontrollable. Uncontrollable factors include, among others, the airway geometry of the patient's respiratory tract and lung and other respiratory diseases. Of the controllable factors, two are of particular interest. The first is the droplet size and droplet size distribution. The second is the breathing pattern.
A major factor governing the effectiveness of drug deposition in the lungs is the size of the inspired particles. Depending on the particle size, total deposition in various regions of the lung may vary from 11% to 98%. See Heyder et al., Aerosol Sci., 1986, 17, 811-825, the disclosure of which is herein incorporated by reference. Therefore, proper selection of particle size provides a way to target liquid droplets to a desired lung region. It is particularly difficult, however, to generate a liquid spray in which all the droplets will have the same size or the same aerodynamic behavior such that drug deposition in the desirable lung region is predictable.
A parameter that may be used to define droplet size is the respirable fraction (RF). The respirable fraction (RF) is defined as the fraction of the mass of aerosol droplets falling between a particular size range, usually in the range from about 1 .mu.m to 6 .mu.m. See D.C. Cipolla, et al., Assessment of Aerosol Delivery Systems for Recombinant Human Deoxyribonuclease, S.T.P. Pharma Sciences 4(1) 50-62, 1994, the disclosure of which is herein incorporated by reference. As used hereinafter, the term respirable fraction (RF) will include the percentage of droplets having sizes falling in the range of from about 1 .mu.m to 6 .mu.m. Another parameter that may be used to evaluate nebulization performance is the efficiency (E). The efficiency (E) of a nebulizer is the amount of liquid which is actually aerosolized and leaves the nebulizer in aerosolized form as compared to the amount of liquid that is initially supplied to the nebulizer. See D.C. Cipolla, et al., Assessment of Aerosol Delivery Systems for Recombinant Human Deoxyribonuclease, S.T.P. Pharma Sciences 4(1) 50-62, 1994. Still another parameter that may be used to measure the performance of nebulizers is the delivery percentage (D) which is the respirable fraction (RF) multiplied by the efficiency (E) . See D.C. Cipolla, et al., Assessment of Aerosol Delivery Systems for Recombinant Human Deoxyribonuclease, S.T.P. Pharma Sciences 4(1) 50-62, 1994.
A variety of inhalation devices have been proposed including air jet nebulizers, ultrasonic nebulizers, and metered dose inhalers (MDIs). Air jet nebulizers usually utilize a high pressure air compressor and a baffle system that separates the small particles from the spray. Ultrasonic nebulizers generate ultrasonic waves with an oscillating piezoelectric crystal to produce liquid droplets. Another type of ultrasonic nebulizer of interest is described in U.S. Pat. Nos. 5,261,601 and 4,533,082. This nebulizer includes a housing that defines a chamber for holding a quantity of liquid to be dispensed. A perforated membrane is held over the chamber and defines a front wall of the chamber, with the rear surface of the membrane being in constant contact with the reservoir of liquid held in the chamber. The apparatus further includes an ultrasonic vibrator connected to the housing to vibrate the perforated membrane. Typical MDIs usually employ a gas propellant, such as CFC, which carries the therapeutic substance and is sprayed into the mouth of the patient.
Most commercially available inhalers produce sprays having a respirable fraction (RF) of 80% or less, with ultrasonic nebulizers usually having a respirable fraction (RF) of less than about 50%, thereby making dosing control difficult and inaccurate. Presently, most commercially available inhalers also have a poor efficiency (E), usually less than about 60%. See D.C. Cipolla, et al., Assessment of Aerosol Delivery Systems for Recombinant Human Deoxyribonuclease, S.T.P. Pharma Sciences 4(1) 50-62, 1994. Such inefficiency often results from the construction of the nebulizer since a certain amount cannot be nebulized and remains within the device. Since most commercially available nebulizers have both a poor respirable fraction (RF) and a poor efficiency (E), the delivery percentage (D) is also poor. Therefore, such inhalers have generally not been used for delivery of drugs that have potent therapeutic agents such as hormones and peptides or other drugs having a high level of toxicity and which can be expensive.
The second factor influencing droplet deposition is the patient's breathing pattern. Inhalation flow rate affects the probability of particle impact, while tidal volume and lung volume affect particle residence time in each lung region. Therefore, effective droplet deposition should be adaptable to the inhalation flow rate as well as the patient's tidal volume and lung volume.
Other important factors often considered when designing an effective therapeutic drug delivery system include both cost and convenience. When nebulizing the medicament, the apparatus involved usually comes in contact with the medicament. Hence, the apparatus will need to be sterilized before reuse, or discarded. However, sterilization may not be convenient for a hand held portable device. Disposal can also be expensive, particularly when the apparatus includes a piezoelectric crystal for nebulizing the liquid.
It would therefore be desirable to provide improved apparatus and methods for the delivery of liquids to the respiratory system. Such apparatus and methods should be capable of producing a spray which may predictably be deposited in selected regions of the lungs. Further, it would be desirable if such a spray were produced from a small volume of liquid. Moreover, it would be desirable if the apparatus and methods provided for a controlled drug delivery rate, preferably being based on the rate of inspiratory air flow generated during inhalation. Finally, it would be desirable if such methods and devices were inexpensive, efficient, and easy to use.
2. Brief Description of the Background Art
U.S. Pat. No. 4,533,082 describes a vibrating orifice apparatus with a multiplicity of apertures for producing liquid droplets.
As previously described, U.S. Pat. No. 5,261,601 describes an atomizer having a membrane covering a liquid chamber.
Apparatus for atomizing liquids such as liquid fuel, water, liquid drugs are described in U.S. Pat. Nos. 3,812,854; 4,159,803; 4,300,546; 4,334,531; 4,465,234; 4,632,311; 4,338,576; and 4,850,534.
D.C. Cipolla, et al., Assessment of Aerosol Delivery Systems for Recombinant Human Deoxyribonuclease, S.T.P. Pharma Sciences 4(1) 50-62, 1994, previously incorporated by reference, describes various inhalation devices and provides selected data on their efficiency (E) and respirable fraction (RF) values.